Wire saws have been known as means for slicing workpieces such as semiconductor ingots into wafers. A wire saw has a wire row formed by winding a slicing wire around a plurality of grooved rollers multiple times, and is configured to slice a workpiece simultaneously at each wire position by causing the slicing wire to travel in the direction of a wire axis at a high speed and feeding the workpiece toward the wire row while supplying a slurry properly.
FIG. 1 schematically shows an exemplary wire saw.
As shown in FIG. 1, the wire saw 1 has a wire 2 to slice a workpiece W. The wire 2 is unreeled from a wire reel 6 on a supply side and enters a group of grooved rollers 3. The wire 2 is wound around the grooved rollers 3 about 300 to 500 times so as to form the wire row. The wire 2 further exits from the group of grooved rollers 3 and is reeled by a wire reel 6′ on a collection side.
When the workpiece W is sliced, axial reciprocating motion having predetermined travel distances is imparted to the wire 2 and a new line of the wire is gradually supplied from the supply side to the collection side.
The amount of wear of the wire wound around the grooved rollers 3 is accordingly larger on the side to which the wire is collected, the collection side, than on the side to which the new line is supplied, the supply side; the diameter of the wire is smaller on the collection side. The thickness of wafers sliced on the collection side is thus apt to be thicker than that of wafers sliced on the supply side.
In view of this, a grooved roller in which intervals between grooves on the collection side are narrower than on the supply side is used (See Patent Document 1).
Incidentally, the wire saw uses a wire composed of a material with high abrasion resistance, high tension resistance, and high hardness such as a piano wire and a grooved roller composed of a resin with prescribed hardness, which prevents damage of the wire. Unfortunately, wear or fatigue of the wire that occurs over time may cause a break in the wire during slicing of a workpiece, making it impossible to continue the slicing of the workpiece.
In this case, after the workpiece fed is disengaged from the wire (a disengage operation), the broken portion of the wire is usually pulled out to a proper position on the outside of one grooved roller by a manual operation or manipulating a grooved roller actuator and the ends of the broken portion are connected. If the broken portion is unusable, this potion is removed and replaced with a new line before the connection. The connected portion of the wire is then pulled again to a position at which the connected portion is not directly involved in the slicing of the workpiece (a pulling operation).
After the wire is repaired in the above manner, each line in the wire row is engaged with a corresponding cut portion of the workpiece (a return operation). The slicing of the workpiece is then resumed to complete the slicing of the workpiece (a restoration operation).
To solve the problem of the break in wires, it is known that wire strength is improved (See Patent Document 2, for example) and when a wire saw detects a symptom of the wire break, slicing is stopped (See Patent Document 3). These technics contribute reduction in the frequency of the wire break.